Steels_ Metallurgy and Applications, Third Edition

372 Steels: Metallurgy and Applications

Automotive exhausts/catalytic converters

Motor car owners were conditioned to the fact that a traditional mild steel
exhaust would need to be replaced after a typical service life of only 18 months,
whereas engine and transmission components are expected to last in excess of
10 years/100000 miles. The use of aluminized mild steel in place of uncoated
material is cost-effective but, even so, the average life of the exhaust is only
extended to about 30 months. On the basis of life-cycle costing, the most effec-
tive material for exhausts is a 12% Cr steel which will give a life of four to five
years at a cost of about 1.5 times that of a mild steel system.
During the 1970s, British Steel directed major effort to the optimization of
high-chromium steels for automotive exhausts following a major survey that
showed that corrosion was responsible for about 80% of the failures that occurred
in mild steel exhausts. Two types of corrosion are operative in automotive
exhausts, namely:

1. That taking place on the outside of systems due to the action of water/solids
   thrown up from road surfaces and, more particularly, from the use of deicing
   salts in winter periods.
   2 That occurring on the inside of exhausts due to the action of corrosive exhaust
   gas condensate which forms in the cooler parts of the system.

Information was available which showed that Type 302 (18% Cr, 8% Ni)
austenitic stainless steel would be very effective in overcoming these corrosion
problems but it was considered that this material would prove too expensive
to achieve a major market conversion from mild steel systems. Additionally,
experience in the United States had indicated that a 12% Cr muffler grade steel
might provide adequate corrosion resistance.
On the basis of preliminary laboratory tests, samples of steels containing up
to 15% Cr were exposed on both the inside and outside of an exhaust system
for 8
89 months, including the winter period, and during this time the car covered
16000 miles. 41 This trial confirmed that a very significant improvement in corro-
sion resistance over mild steel could be achieved in steels containing 10-12% Cr
and therefore further work was focused on steels within this chromium range.
This culminated in the development of Hyform 409, a variant of the standard
titanium-stabilized Type 409 grade, with the following typical composition: 42

0.02% C, 0.6% Si, 0.3% Mn, 11.4% Cr, 0.4% Ti

Because the manufacture of exhausts involves major welding operations, a stabi-

lized grade was required in order to eliminate any possibility of intergranular

corrosion. However, as indicated earlier, the addition of titanium to a 12% Cr

steel also ensures that the microstructure is ferritic, thereby providing a much

more formable and weldable material than the 12% Cr martensitic grades.

Hyform 409 can be welded satisfactorily using most techniques, including

MIG, TIG and HF. When MIG welding is employed, the preferred filler metal is